#2009 The Society of Polymer Science, Japan Reassessment of Free-Radical Polymerization Mechanism of Allyl Acetate Based on End-Group Determination of Resulting Oligomers by MALDI-TOF-MS Spectrometry By Akira MATSUMOTO,Ã Takeo KUMAGAI, Hiroyuki AOTA, Hideya KAWASAKI, and Ryuichi ARAKAWA Allyl monomers polymerize only with difficulty and yield polymers of medium-molecular-weight or oligomers. This is attributable to ‘‘degradative monomer chain transfer.’’ However, the well-known allyl polymerization mechanism is based on only the kinetic data but any structural identification is not given. Allyl acetate (AAc), a most typical allyl monomer, was polymerized radically and the resultant oligomeric poly(AAc)s were characterized using MALDI-TOF-MS spectrometry in order to reassess the AAc polymerization mechanism proposed by Litt and Eirich. The induced decomposition of benzoyl peroxide by both growing polymer radical and monomeric allyl radical was presumed but it was never of importance. Then, the fate of resonance-stabilized monomeric allyl radical generated via monomer chain transfer of growing polymer radical was pursued in terms of the competition between the initiation of a new polymer chain and the chain stopping reaction of a growing polymer radical providing monomeric allyl groups as the initial and terminal end-groups, respectively. The À2 monomer chain transfer constant was estimated to be 3:73 Â 10 from the Pn value of poly(AAc) obtained at a low initiator concentration where the coupling termination of growing polymer radical with monomeric allyl radical was negligible. KEY WORDS: Allyl Acetate / Allyl Radical / Allyl Polymerization / Degradative Monomer Chain Transfer / MALDI-TOF-MS / End-Group Determination / The free-radical polymerization of allyl monomers, gener- deuteriums showed a rate more than twice that for normal ally referred to as ‘‘allyl polymerization,’’ has received much AAc and yielded a proportionately higher molecular weight less attention than the corresponding vinyl polymerization of product, due to the stronger C-D bond than the C-H bond. common vinyl monomers.1–4 Thus, the polymerization of allyl Concurrently, a series of allyl monomers have been polymer- acetate (AAc) initiated by benzoyl peroxide (BPO), studied by ized by Gaylord and Eirich.7 Sakurada and Takahashi also Bartlett and Altschul5 in the early 1940s, was the first topic of made a detailed study of the polymerization of allyl detailed investigations into allyl polymerization. The low monomers and proposed a kinetic mechanism for it.8,9 As a tendency to polymerize and the obtainment of products with summary of the results reported in the period of 1940’–1960, a low degree of polymerization were characteristic of allyl Litt and Eirich10 developed a kinetic scheme that would polymerization so that a large amount of initiator was required, account quantitatively for the facts of allyl polymer- i.e., reaching up to almost 102 times when compared with ization, especially focused on ‘‘degradative monomer chain common vinyl polymerization. These results are attributable transfer.’’ to ‘‘degradative monomer chain transfer.’’ That is, the hydro- Therefore, the most characteristic feature of the polymer- gen atoms attached to the carbon atom alpha to the double ization of allyl monomers is of the significance of monomer bond, i.e., allylic hydrogens are responsible for this chain chain transfer which competes with normal propagation, transfer: yielding polymers of medium-molecular-weight or oligomers, not really low molecular weight, and produces resonance- MþCH =CH-CH -R ! P þ½CH ÁÁÁ{CHÁÁÁ{CH{R 2 2 2 stabilized monomeric allyl radical, causing the allyl monomers ð Þ monomeric allyl radical to retard polymerization. In this connection, another important The resulting monomeric allyl radical is less active and/or has aspect of the characteristics of allyl polymerization should be less tendency to initiate a new polymer chain because of an pointed out as follows: In the radical polymerization of allyl ability to stabilize itself by resonance. Therefore, the mono- monomers, the research objectives would be oligomers or meric allyl radicals undergo termination by reaction with each medium-molecular-weight molecules, the molecular weights of other or with growing polymer radicals, the monomer chain which are essentially intermediate between low- and high- transfer being essentially a termination reaction. This monomer molecular-weight molecules. Accordingly, allyl polymeriza- chain transfer was further confirmed by Bartlett and Tate;6 the tion would provide useful information for finding out the polymerization of a deuterium-substituted AAc with allylic inherent phenomena of polymers, not being of real low- Department of Chemistry and Materials Engineering, Faculty of Chemistry, Materials and Bioengineering & High Technology Research Center, Kansai University, 3-3-35 Yamate-cho, Suita 564-8680, Japan ÃTo whom correspondence should be addressed (Tel: +81-6-6368-0936, Fax: +81-6-6339-4026, E-mail: [email protected]). 26 Polymer Journal, Vol. 41, No. 1, pp. 26–33, 2009 doi:10.1295/polymj.PJ2008174 Reassessment of Polymerization Mechanism of Allyl Acetate molecular-weight molecules, by pursuing the molecular weight dependences of various properties of resulting medium-mo- EXPERIMENTAL lecular-weight molecules. For example, anomalous head-to- head addition occurred remarkably11,12 and its chain length Materials dependency was observed due to the steric effect of side AAc (Tokyo Chemical Industry Co., Ltd., Tokyo) as a chains.13 monomer was purified by distillation. 2,2-Azobis(isobutyroni- Here it is worthy to note that other monomers such as trile) (AIBN) and 1,10-azobis(cyclohexane-1-carbonitrile) methyl methacrylate and methacrylonitrile, which also contain (ACHN) (azo initiators; Wako Pure Chemical Industries, allylic C-H bonds, do not undergo extensive monomer chain Ltd., Osaka) and BPO (NOF Corp., Aichi) as initiators were transfer, and thus form high-molecular-weight polymers. This purified by conventional methods. is ascribed not only to lower reactivity of the growing polymer 2,5-Dihydroxybenzoic acid (DHBA) (Sigma-Aldrich, radical formed from these conjugated monomers but also to Milwaukee, USA) as a matrix, sodium iodide (NaI) (Wako higher reactivity of their C=C double bonds.1 In contrast to the Pure Chemical Industries, Ltd., Osaka) as a cationizing agent, above conjugated monomers, allyl monomers have typical and polyethylene glycol (PEG, Mw ¼ 2000 Da) (Wako Pure unconjugated double bonds. In this connection, the Q and e Chemical Industries, Ltd., Osaka) were purchased. values of one allyl group of diallyl phthalate (DAP) as a typical multiallyl monomer were estimated to be 0.029 and 0.04, Polymerization respectively, by the copolymerization of DAP with various Polymerization was carried out in a glass ampule containing vinyl monomers;14,15 the low Q value of 0.029 demonstrates the required amounts of monomer, initiator, and solvent. The that DAP is a typical unconjugated monomer. This uncon- ampule was degassed three times by the usual freezing and jugation of allyl monomer is significant to understand the thawing technique under a vacuum and then sealed off. It was characteristics of allyl polymerization. That is, the low then placed in a thermostat regulated at a required temperature. reactivity of C=C double bond in unconjugated monomer After a definite reaction time, the reaction mixture was synergizes with the low reactivity of resonance-stabilized subjected to vacuum-evaporation at room temperature in order monomeric allyl radical to lead less tendency to initiate a new to remove unreacted monomer. It should be noted that only polymer chain and, on the contrary, the high reactivity of methanol soluble oligomers were obtained. growing polymer radical formed from unconjugated monomer promotes the allylic hydrogen abstraction. On the other hand, Measurements Sakurada8,9 and Gaylord16 have pointed out that although the MALDI-TOF-MS spectra were acquired in positive reflec- degradative monomer chain transfer is quite usual in allyl tion mode using an Axima-CFR time-of-flight mass spectrom- polymerization, monomeric allyl radicals also have an ability eter (Shimadzu/Kratos, Manchester, UK) with a pulsed N2 of adding monomer, and thus of continuing the chain reaction, laser (337 nm). Poly(AAc) (2.0 mg/mL), DHBA matrix (20 referred to as ‘‘effective monomer chain transfer.’’ Further- mg/mL), and NaI cationizing agent (1.0 mg/mL) were dis- more, Gaylord17 reported the radical displacement as a chain- solved in THF. 0.5 mL each of these solutions was deposited end forming reaction by effective monomer chain transfer, on a stainless sample target by the overlayer method as i.e., -scission of growing polymer radical through addi- follows: first deposition, matrix solution; second, cationizing tion-fragmentation transfer as is discussed later, the product agent solution; third, polymer solution. The analyte ions were radical being active enough to initiate a new polymer accelerated at 20 kV under delayed extraction conditions. chain. Sodiated ions of PEG (Mw ¼ 2000 Da) were used for calibra- Although the free-radical polymerization of various allyl tion. monomers is specific as mentioned above, it may lack of direct SEC measurements were carried out at 40 C in THF using evidence for a full mechanistic discussion, especially including three Shodex KF-803L columns at polymer concentrations of initiation and termination reactions. Here it is worthy to